The airborne/spaceborne imaging spectrometer is an effective remote sensor for the earth sciences. There are two types of airborne multi-spectral imaging instruments. One uses a set of fixed wavelength bandpass filters to obtain spectral information. These instruments have provided a wealth of extremely useful information, but they suffer either from limited spectral coverage due to the limited number of filters that can be designed into a camera or from inadequate spectral resolution. The other type of instrument uses a scanning image technique with dispersive devices, such as diffraction gratings to provide spectral information. Some of these instruments can provide high spectral resolution observations. However images are composed later in the laboratory. Thus, the operation is not real-time. One notable example of this type of instrument is the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS). There is therefore an ongoing need for an airborne/spaceborne imaging spectrometer which can operate in real-time and which does not require a large number of filters or suffer from inadequate spectral resolution. It will be seen hereinafter that the present invention overcomes these disadvantages and meets such a need by utilizing an acousto-optic tunable filter (AOTF) in an imaging spectrometer. An AOTF is a solid state electronically tunable spectral bandpass filter which operates on the principle of acousto-optic interaction in an anisotropic medium. Several features of AOTFs make them attractive as a new generation of wavelength sorters for airborne/spaceborne imaging spectrometers. They are fast devices providing electronic tuning in microseconds. They have a relatively wide wavelength range of operation. They can provide a large throughput. They have high resolution. They can be operated in a sequential, random and multi-wavelength access modes. They are compact, rugged and lightweight in construction and have an all solid state structure without moving parts In addition, AOTFs can operate from visible to infrared wavelengths with spectral resolutions ##EQU1## ranging from approximately 10.sup.2 to 10.sup.4. Thus, the AOTF is suitable for observations of detailed spectral signatures on earth and planetary surfaces. Furthermore, because of diffraction in an anisotropic medium, there are two diffracted monochromatic beams with polarization orthogonal to each other. Therefore, the AOTF provides a unique capability for measuring spectral, spatial and polarimetric properties of the incoming light, simultaneously with a single instrument.
The use of an AOTF for spectroscopic applications per se, is not new. The following U.S. Patents describe the use of an AOTF in a spectroscopic application.
U.S. Pat. No. 4,883,963 to Kemeny et al is directed to a spectrometer having an acousto-optic tunable filter with programmable rapid random wavelength access for analyzing changing or moving samples of solid or liquid material. The system has the AOTF filter and optical system including polarizing elements, fiber optics for remote detection, if necessary, and control electronics in a computer. The detector however is a semiconductor, not a CCD and the disclosure is not directed to an imaging spectrometer.
U.S. Pat. No. 4,663,961 to Nelson et al is directed to a system for remote chemical analysis using an AOTF. The system has an AOTF, a microcomputer for control and data handling, an optical system with a fiber optics cable for remote use. The detector is lead selenide, not a CCD and there is no reference to imaging.
U.S. Pat. No. 4,622,845 to Ryan et al is directed to a portable system for detecting and measurement of gases using an AOTF. The portable system uses the AOTF, data reduction and control, and a photo acoustic detector. The acoustic detector is only useful for gas analysis and not for imaging.
U.S. Pat. No. 4,652,756 to Ryan et al is directed to an automated AOTF analyzing system for monitoring stack emissions. The system has an optical system, AOTF optics and detector, but no polarizing elements. Discrimination against extraneous radiation is provided by electronics with a microcomputer operating the AOTF in a pulsed mode. The system could be used for solid or liquid analysis but not for imaging.
U.S. Pat. No. 4,490,845 to Steinbruegge et al is directed to an automated IR analyzer using an AOTF. The system comprises two major sub-systems, optics and electronics and is a broadband solid state spectrometer adapted for gases. The optic sub-system has an AOTF detector and polarizing elements. The electronic sub-system has a microcomputer and control electronics. This system, directed to gas analysis, could be used on solid state or liquid samples but not for imaging.
The following non-patent prior art references also disclose relevant subject matter:
1. "Acousto-Optic Devices and Applications" by I. C. Chang, IEEE Transactions On Sonics and Ultrasonics, Vol. SU-23, No. 1, January 1976;
2. "Tunable Acousto-Optic Filters: An Overview" by I. C. Chang, Optical Engineering, Vol. 16, No. 5, September/October 1977;
3. "Acousto-Optic Image Filtering" by I. B. Belikov et al, Soviet Tech. Phys. Lett. 10(10), October 1984;
4. "Rapid Scanning Fluorescence Spectroscopy Using an Acousto-Optic Tunable Filter" by Ira Kurtz et al, Rev. Sci. Instrum., 58(11), November 1987.